The present invention relates to frequency measurement devices of the type which measure the number of cycles of the signal to be measured within a fixed sampling period.
In the measurement of high frequency alternating signals, such as rf, satisfactory accuracy can usually be attained by counting the positive-going or negative-going zero crossings of the alternating signal over a fixed sampling period. For example, a 27 megahertz signal can be very accurately measured by counting the number of positive-going or negative-going zero crossings of the signal occurring in one second. This is the principle employed in commercially available frequency measurement equipment manufactured and sold by numerous instrument makers.
In the conventional frequency measurement equipment, the beginning and end of the sampling period occur at arbitrary points in the phase of the signal being measured. If the beginning of the sampling period just misses a zero crossing, the equipment counts almost a full cycle less than the actual number of cycles in the sampling period. Similarly, if the end of the sampling period occurs just before a zero crossing, the equipment again counts almost a full cycle less than the actual number of cycles in the sampling period. It is, therefore, possible for conventional frequency measurement equipment to count almost two complete cycles less than the actual number of cycles during the measurement period. However, when measuring signals of high frequency such as 27 kilohertz during long periods of time such as one second, the loss of two cycles from 27,000 cycles results in an insignificant error in the measured quantity.
The same is not true when relatively infrequent signals are to be measured over a sampling period that is on the same order of magnitude as the period of the frequency to be measured. For example, frequency measurement devices associated with vehicle wheel speed sensors have a limited capability to generate a rapidly varying periodic signal. For example, a truck wheel speed sensor for 20-inch truck wheels generating 60 pulses per rotation of the wheel produces a frequency of only about 12 hertz per mile per hour of wheel speed. Consequently, a speed of 5 miles per hour produces a frequency of only about 60 hertz. An error of nearly two cycles in a sampling period of 0.04 seconds, for example, yields a measurement error of 33%. As a further complication, it requires only a slightly higher frequency within the sampling period to permit the zero crossings at the beginning and end of the sampling period to be counted. This can cause an almost instantaneous change in apparent speed of about 33%. Since wheel speed measurement devices are typically used as input sensors for wheel slip control systems which interpret velocity changes as indications of wheel skidding and generate brake-release signals in response thereto, such virtually instantaneous changes in measured wheel speed are unacceptable.
In order to improve their accuracy, counter types of frequency measurement equipment must count input cycles for a longer period. To achieve an accuracy improvement of a factor of eight, for example, the sample period must be increased in length by a factor of eight. This requirement conflicts with the desire in, for example, wheel slip control systems, to obtain frequent measurements of wheel speed in order to permit rapid response to changing wheel slip conditions. Practical wheel slip control systems require at least several measurements of wheel speed per second and preferably from about 10 to about 30 measurements of wheel speed per second. With the frequency numbers previously described, it is clear that the partial cycles of input signal cannot safely be ignored but must somehow be accounted for.
The prior art discloses many varieties of phase-locked loop systems, typically employing digital circuits, for generating a signal proportional to the frequency of the relatively slowly occurring input signals. In U.S. Pat. Nos. 4,040,677; 4,047,766; 4,033,633 and 3,838,889, a comparison of an internally controlled frequency pulse generator with the frequency of the incoming signal generates an error signal which causes a number stored in a register to increase or decrease depending upon the sign of the error. The stored number is used to alter the frequency of the controlled frequency pulse generator to attempt to maintain its frequency in step with the sensor input frequency. The correction number stored in the register provides a measure of the frequency of the incoming signal and is passed on to using circuits, typically wheel slip control circuits.
In U.S. Pat. No. 4,056,287, the phase-locked loop idea is employed in which a count-up counter counts up at a frequency varying as the positive exponential with exponent greater than one and a second counter which is triggered on by a predetermined count in the first counter counts down in a second exponential ratio with an exponent inversely proportional to the exponent in the first counter. The use of exponential or logarithmic counting sequences is proposed as a method of rapidly attaining a measurement of the input frequency.
All of the digital phase locked loop devices require a large number of digital circuits to perform all of the complex functions involved. Such complexity adds to the manufacturing and maintenance cost of the devices.